In normally aspirated two and four cycle I.C. engines the basic combustion process is as follows. The air-fuel mixture is drawn into the engine through the carburetor due to the low pressure created by the ascending or descending piston depending on two and four cycle. The controlled air-fuel mixture is then compressed by the rising piston in the cylinder to a desirable compression ratio determined by the fuel. The compressed gases are ignited through a spark plug located in the cylinder head before top dead center (TDC) resulting in a sharp increase in temperature and pressure inside the combustion chamber. The expanding gases push the piston down which in turn gets the crank rolling and storing the energy in a flywheel to do useful work.
Ultimately, the flame velocity and degree of combustion have a direct bearing on the a) power output, b) efficiency of engine, c) fuel consumption, d) emission, e) operating temperature, f) sound and vibration levels and g) reliability. The flame velocity and degree of combustion are directly related to the state of turbulence in the charge prior to ignition.
In existing combustion chambers designs in I.C. engines, the combustion chamber is the enclosed space within the cylinder, the cylinder-head and above the piston where burning of charge occurs. The combustion chambers play a vital role in engine characteristics. Since the inception of the I.C. engine, a lot of research and development has been carried out to perfect the combustion chamber to achieve maximum engine efficiency and reliability. The trend in combustion chamber design has been to direct the expanding forces caused due to combustion towards the piston crown and to avoid the dissipation of these forces in the direction that do not produce power.
Two stroke combustion chambers, due to their relatively simple layouts, have evolved and revolved around hemispherical layouts with a center or offset spark plug location since their inception. Four stroke combustion chambers of the early types featured side valves layouts with their large volume low compression cylinder heads prone to detonation and low power outputs.
The most notable research on combustion chambers in the early days was done by Sir Harry Recardo, who enlightened the world about the causes of Detonation and Pinging. Recardo discovered Pinging and Detonation arose through uncontrolled instantaneous combustion occurring in pockets of fuel in the extreme ends of the combustion chamber due to the extreme heat and pressure build up. Ricardo's solution was to concentrate the greater part of the clearance volume over the side valves layout and reducing greatly the clearance between the larger part of the combustion chamber which extended over the piston crown. In the Ricardo layout, the space between the piston and the cylinder head was so small and the surface so cool in relation to the combustion temperatures that the gases trapped in this "Quenched" area did not detonate in the combustion cycle under load. This was an improvement over other combustion chambers. Later over-head valve (O.H.V) layouts gained popularity due to several advantages and attained higher power outputs and sustained reliability. The shape and sizes of four stroke combustion chambers with their overhead valves layouts went through many design changes over the years.
The four stroke combustion chamber layouts evolved through the plain cylindrical form with the required clearance volume, the bath tub type, the wedged shape type, and the hemispherical cross flow type. The hemispherical combustion chamber or hemi-head provides room to accommodate larger valves increasing volumetric efficiency and permits centrally located spark plug which contribute to more efficient combustion, better heat dissipation and higher thermal efficiency.
The concept of a portion of the combustion chamber at close proximities to the piston crown at TDC came to be known as "squish" area or "squish" band earlier referred to as quenched area. In principle, the trapped charge between the piston crown and the squish area nearing TDC starts to be injected towards the main scoop of the combustion chamber causing turbulence prior to ignition greatly reducing detonation and pinging. Higher compression ratios are possible with squish bands resulting in improved engine efficiencies. Turbulence in the charge is also caused by inlet ports, their shapes, angles and surface finish. They greatly help to keep the air-fuel mixture bonded and in a homogeneous state at the point of entry only. Multipoint fuel injection basically achieves very fine break ups of fuel particles prior to entry on the intake stroke and achieves better combustion due to the ideal state of the charge.
Two stroke engines have lesser volumetric efficiency due to the obstruction in the ports and short time/area available in the intake and transfer phase. Due to the size, shape and angles of the ports the charge is in a higher state of turbulence entering the cylinder than four strokes and requires far lesser ignition advance to operate efficiency irrespective to combustion chamber design. Four strokes require higher degree of ignition advance and assisted by vacuum advance to operate efficiently due to the lower state of turbulence and a denser charge before combustion. The turbulence inside the cylinder and head mainly helps to maintain the air-fuel mixture in a gaseous state and prevent condensation of fuel droplets preventing erratic and incomplete combustion. In recent times the most accepted practice to create turbulence is to provide squish bands in the combustion chamber.
The squish area are normally placed in the outer circumference of the combustion chamber and are machined smooth. The squish area could be a band or a tapered area or two bands on opposite sides. The squish area are either flat or angled depending on the profile of the piston crown. They are machined smooth to a high degree of finish and set up in design with a close tolerance between combustion chamber and piston at TDC preventing contact.
In principle, the piston on the upward stroke causes the compression to progressively increase. Nearing TDC, the gases around the squish band and the piston crown are pushed towards the center scoop causing Turbulence which in turn improve flame propagation as ignition has occurred before TDC and greatly reduces Pinging and Detonation. Thus, present day two stroke combustion chambers are hemispherical or the "top hat" type with a circular or partial squish band and are machined smooth with no sharp edges. The spark plugs are located centrally or offset depending on the requirement. They are made of alloys of aluminum of high conductivity and, in certain cases, are water cooled.
Present day four stroke combustion chambers house the inlet and exhaust valves. Multiple valve layouts are standard feature in high performance design. Partial or circular squish bands are provided and are finished smooth to a high degree with no sharp edges. The spark plug is location depends on design and availability of space. In the case of aircraft engines, twin plugs are mandatory. Cylinder heads are largely made of alloys of aluminum having steel inserts for valve seats and water cooled in most cases. Basic designs typically are bath tub, wedged or double wedged with a flat roof or hemispherical cross flow type with inclined valve layouts.
Over the last 60 years standard practice is to have a squish area of 20% to 40% or more of the combustion chamber area either concentric or offset to the cylinder axis at close proximities of the piston crown, causing turbulence in two stroke engines. Depending on the number of valves and layouts, four stroke combustion chambers are machined to provide the squish area resulting in a puff of mixture pushed towards the spark plug causing turbulence resulting in better combustion.
In either case the surface of the combustion chamber, squish bands and the piston crown are normally machined smooth with a high degree of finish with the right tolerance to prevent contact at TDC on existing two and four cycle engines in production.
Compared to diesel engines (with their higher efficiencies), the present day combustion chamber layouts in two and four cycle petrol (gasoline) engines include the following design defects and limitations. First, diesel engines operate at higher efficiencies due to the turbulence caused by direct diesel injection into the combustion chamber before TDC. Second, the diesel also burns more completely due to the turbulence created by the high pressure spray resulting in lesser emissions and unburnt fuel. Third, the diesel has higher resistance to flash point due to its composition and hence can withstand much higher compression ratios than petrol or kerosene. Fourth, the petrol and kerosene engines have a threshold on compression ratios due to its properties and lower flash points compared to diesel. Fifth, the petrol and kerosene need to be atomized with air to form a homogenous mixture before it is drawn into the cylinder, as compared to the diesel which is injected prior to ignition directly into the combustion chambers. Sixth, as compression is applied the air-fuel mixture tends to get unstable and starts to separate and condense causing erratic and incomplete combustion. Seventh, the only possible method to keep the mixture in a homogeneous state is to induce turbulence prior to ignition. Eighth, the only method known to cause turbulence are squish bands or squish areas located in the combustion chambers which help retain the air-fuel mixture. Ninth, squish bands have their disadvantages too. They prevent total combustion as fuel trapped between the squish band are less volatile due to the lower temperatures caused by masking. Tenth, squish bands and compression ratios have their limitations on creating turbulence, often resulting in heat build up due to uneven thickness of metal in the squish band resulting in detonation and pinging under load. Eleventh, very often at lower operating speeds incomplete combustion occurs causing excessive emissions and poorer torque compared to diesel engines. Twelfth, leaner air-fuel mixture result in slower flame velocities resulting in excessive heat build up causing emissions of oxides of nitrogen. Thirteenth, in two cycle engines the combustion temperature builds up very rapidly due to the short intervals of combustion occurring each revolution. Hence compression ratios are critical and cannot be increased to four stroke parameters. Fourteenth, the charge comprising of petrol/air drawn in the induction stroke is invariably preheated due to engine temperatures and further heated by compression bringing it to a critical state before ignition. Fifteenth, carbon deposits in the combustion chamber absorb heat and cannot dissipate the heat into the combustion chamber and eventually contribute to preignition and detonation and auto ignition once the engine is switched off. Sixteenth, under load, lean burn, high compression engines require very careful monitoring of air-fuel ratios and ignition timing to avoid pinging and detonation resulting in excessive emissions. Seventeenth, there are limits to which a petrol I.C. engine could stand up to. Exceeding these limits the existing combustion chamber design cannot cope with the following parameters: a) temperature build up during combustion cycle resulting in detonation and pinging under load; b) squish bands greatly reduce detonation and pinging, but cause unburnt fuel and excessive emissions; c) carbon build up in combustion chambers and piston crown build up compression ratios and largely contribute to auto ignition and erratic and noisy running resulting in excessive emissions; d) richer mixture bring down combustion chamber temperature but result in excessive carbon monoxide and Carbon emissions; e) leaner settings result in low flame velocities and higher combustion temperatures due to time lag, causing emissions of oxides of nitrogen.